Method and system for sampling and determining the presence of compounds

ABSTRACT

A system and method are provided for minimizing the effects of background signals in masking signals indicating the presence of substances to be detected such as contaminants in materials moving rapidly along a conveyor. The contaminants detected may include nitrogen containing compounds and hydrocarbons. The system and method of the present invention minimize during detection of the presence or absence of such substances, the number of falsely positive indications of the presence of such substances due to background signals and changes in background signals. The substances detected are divided into first and second sample portions and the respective portions are heated. The first heated portion is mixed with ozone to cause a chemical reaction therewith in order to generate radiation by chemiluminescence having characteristic wavelengths related to substances in the first portion. The second portion heated is also mixed with ozone to cause a chemical reaction therewith in order to generate radiation by chemiluminescence having characteristic wavelengths related to substances in the second portion. The radiation of the respective first and second portions is selectively detected. The heating and detecting steps are performed in a manner so as to yield a higher level of detected radiation from one of the portions of the sample then the other for at least some of the selected compounds being detected. Electrical signals from the respective first and second portions are generated and compared in order to determine the presence or absence of selected compounds in the sample. Appropriate reject signals for a bottle sorting system are generated accordingly.

This application is a continuation-in-part of application Ser. Nos.07/890,863 filed Jun. 1, 1992 and 07/890,864 filed Jun. 1, 1992, both ofwhich are assigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION

The present invention relates to an inspection system for sampling anddetermining the presence of certain substances, such as residues ofcontaminants within containers such as glass or plastic bottles. Morespecifically, the present invention relates to an improved sampling andanalyzing system and method for determining the presence of substancessuch as residues of contaminants, as in containers such as beveragebottles rapidly moving along a conveyor past a test station in acontainer sorting system.

In many industries, including the beverage industry, products arepackaged in containers which are returned after use, washed andrefilled. Typically refillable containers, such as beverage bottles, aremade of glass which can be easily cleaned. These containers are washedand then inspected for the presence of foreign matter.

Glass containers have the disadvantage of being fragile and, in largervolumes, of being relatively heavy. Accordingly, it is highly desirableto use plastic containers because they are less fragile and lighter thanglass containers of the same volume. However, plastic materials tend toabsorb a variety of organic compounds which may later be desorbed intothe product thereby potentially adversely affecting the quality of theproduct packaged in the container. Examples of such organic compoundsare nitrogen containing compounds such as ammonia, organic nitrogencompounds, and hydrocarbons including gasoline and various cleaningfluids.

The aforementioned application Ser. No. 07/890,863 discloses a systemand method for detecting the presence of these nitrogen containing andhydrocarbon compounds using a chemiluminescence analyzer. That systemand method works quite well, but improvements are desirable to overcomeinterferences which may occasionally cause difficulties in achievingdesired sensitivity and accuracy of detection. Such interferences resultfrom background signals which may mask detection of low levels ofcertain compounds and whose variation with time may also result in falsepositives (and thus unwarranted rejection of uncontaminated containers).Accordingly, a need in the art exists for a chemiluminescence analyzerwith improved accuracy and sensitivity.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea method and system for detecting the presence or absence of specificsubstances--e.g., contaminants such as nitrogen containing compounds andhydrocarbons, in materials as the materials move rapidly along aconveyor with improved accuracy which minimizes the deleterious effectsof background signals.

It is a particular object of the invention to minimize the effects ofbackground signals in masking signals indicating the presence of suchsubstances to be detected.

It is another particular object of the invention to minimize, duringdetection of the presence or absence of such substances, the number offalsely positive indications of the presence of such substances due tobackground signals and changes in background signals.

The objects of the present invention are fulfilled by providing a methodcomprising the steps of:

collecting a sample;

dividing the sample into first and second portions;

heating the first portion of the sample to a first temperature;

heating the second portion of the sample to a second temperature;

mixing the heated first portion of the sample with ozone to cause achemical reaction therewith in order to generate radiation bychemiluminescence having characteristic wavelengths related tosubstances in said first portion;

mixing the heated second portion of the sample with ozone to cause achemical reaction therewith in order to generate radiation bychemiluminescence having characteristic wavelengths related tosubstances in said second portion;

selectively detecting radiation emitted by chemiluminescence from thefirst portion of the sample;

selectively detecting radiation emitted by chemiluminescence from thesecond portion of the sample;

said heating and detecting steps being performed in a manner so as toyield a higher level of detected radiation from one of said portions ofthe sample than the other for at least some of the selected compounds;

generating first electrical signals from the radiation selectivelydetected from the first portion of the sample and second electricalsignals from the radiation selectively detected from the second portionof the sample; and

comparing the first electrical signals with the second electricalsignals in order to determine the presence or absence of selectedcompounds in the sample;

said heating, mixing, detecting, and generating, steps for said firstportion being performed at essentially the same time as said heating,mixing, detecting, and generating steps for said second portion, andsaid comparing step being performed in a manner so as to cancelbackground signals in said portions.

In a preferred embodiment heating of the first portion is performed in afirst converter having a ceramic heating chamber and heating of thesecond portion is performed in a second converter having nickelmaterials in its heating chamber. Therefore, the respective first andsecond sample portions are oxidized in different chemical environments.The radiation generated by chemiluminescence of the sample in theceramic converter is passed through a quartz filter and detected. Theradiation generated by chemiluminescence of the sample in the nickelconverter is passed through a red (infrared) filter and detected. Thesignal related to radiation passing through the quartz filter issubtracted from the signal related to the radiation passing through thered filter by a computer. The result is compared to certainpredetermined threshold criteria to determine the presence or absence ofcertain nitrogen or hydrocarbon compounds of interest. Appropriatereject signals for a bottle sorting system are generated accordingly.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are not limitingof the present invention and wherein:

FIG. 1 is a schematic block diagram of the sampling and residueanalyzing system disclosed in U.S. application Ser. No. 07/890,863illustrating a plurality of containers moving seriatim along a conveyorsystem through a test station, reject mechanism and washer station;

FIG. 2 is a block diagram also disclosed in U.S. application Ser. No.07/890,863 illustrating a possible implementation of the system of FIG.1 in a detector system in which the contaminant being detected may be anitrogen containing compound;

FIG. 3 is a schematic diagram of an improved analyzer system accordingto the present invention;

FIG. 4 illustrates graphs of signals detected and processed by thesystem of FIG. 3 for a sample having nitrogen containing compounds suchas NH₃ ;

FIG. 5 illustrates graphs of signals detected and processed by thesystem of FIG. 3 for a sample containing hydrocarbons such as dieselfuel or kerosene;

FIG. 6 illustrates graphs of signals detected and processed by thesystem of FIG. 3 for samples containing an unknown contaminant affectedin a similar manner by heating in two different converters; and

FIG. 7 illustrates graphs of signals detected and processed by thesystem of FIG. 3 for samples containing no contaminants of the typedetectable by the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The disclosures of the aforementioned applications Ser. Nos. 07/890,863and 07/890,864 are incorporated herein by reference.

With reference to FIG. 1 of parent application Ser. No. 07/890,863,there is illustrated a conveyor 10 moving in the direction of arrow Ahaving a plurality of uncapped, open-topped spaced containers C (e.g.,plastic beverage bottles of about 1500 c.c. volume) disposed thereon formovement seriatim through a test station 12, reject mechanism 28 andconveyor 32 to a washer system. To achieve higher test rates containersC could be touching each other rather than spaced. The contents ofcontainers C would typically include air, volatiles of residues ofcontaminants, if any, and volatiles of any products such as beverageswhich had been in the containers. An air injector 14 which is a sourceof compressed air is provided with a nozzle 16 spaced from, but alignedwith, a container C at test station 12. That is, nozzle 16 is disposedoutside of the containers and makes no contact therewith. Nozzle 16directs compressed air into containers C to displace at least a portionof the contents of the container to thereby emit a sample cloud 18 to aregion outside of the container being tested.

As an alternative to compressed air, CO₂ gas could be utilized as theinjected fluid. Also, the compressed air or CO₂ gas could be heated toenhance volatility of the compounds being tested.

The column of air injected through nozzle 16 into a container C would betypically of the order of about 10 c.c. for bottle speeds of about 200to 1000 bottles per minute. A rate of 400 bottles per minute ispreferable and is compatible with current beverage bottle fillingspeeds. The desired test rate may vary with the size of the bottlesbeing inspected and filled. Of course the bottles could be stationary ormoving slower than 200 bottles per minute and the system would stillwork. Only about 10 c.c. of the container contents would be displaced toregions outside of the bottle to form sample cloud 18.

Also provided is an evacuator sampler 22 which may comprise a vacuumpump or the like coupled to a sampling tube or conduit 20. The tube ismounted near, and preferably downstream (e.g., about 1/16 inch) of theair injector 14 so as to be in fluid communication with sample cloud 18adjacent to the opening at the top of containers C.

Neither nozzle 16 nor tube 20 contacts the containers C at test station12; rather both are spaced at positions outside of the containers inclose proximity to the openings thereof. This is advantageous in that nophysical coupling is required to the containers C, or insertion ofprobes into the containers, which would impede their rapid movementalong conveyor 10 and thus slow down the sampling rate. High speedsampling rates of from about 200 to 1000 bottles per minute are possiblewith the system and method of the present invention. The conveyor 10 ispreferably driven continuously to achieve these rates without stoppingor slowing the bottles down at the test station.

A bypass line 24 is provided in communication with the evacuator sampler22 so that a predetermined portion (preferably about 90%) of the samplefrom cloud 18 entering tube 20 can be diverted through bypass line 24.The remaining sample portion passes to a residue analyzer 26, whichdetermines whether specific substances are present, and then isexhausted. One purpose of diverting a large portion of the sample fromcloud 18 is to reduce the amount of sample passing from evacuatorsampler 22 to residue analyzer 26 in order to achieve high speedanalysis. This is done in order to provide manageable levels of samplesto be tested by the residue analyzer 26. Another purpose for diverting aportion of the sample is to be able to substantially remove all ofsample cloud 18 by evacuator 22 from the test station area and divertthe excess through bypass line 24. In a preferred embodiment the excessportion of the sample passing through bypass line 24 returned to airinjector 14 for introduction into the subsequent containers moving alongconveyor 10 through nozzle 16. However, it would also be possible tosimply vent bypass line 24 to the atmosphere.

It should be understood that sample cloud 18 could be analyzed in situwithout transporting it to a remote analyzer such as 26. It could alsobe transported to analyzer 26 by blowing rather than sucking.

A microprocessor controller 34 including an analog to digital converteris provided for controlling the operation of air injector 14, evacuatorsampler 22, residue analyzer 26, a reject mechanism 28 and an optionalfan 15. Container sensor 17 including juxtaposed radiation source andphotodetector is disposed opposite a reflector (not shown) acrossconveyor 10. Sensor 17 tells controller 34 when a container arrives atthe test station and briefly interrupts the beam of radiation reflectedto the photodetector. Optional fan 15 is provided to generate an airblast towards sample cloud 18 and preferably in the direction ofmovement of containers C to assist in the removal of sample cloud 18from the vicinity of test station 12 after each container C is sampled.This clears out the air from the region of the test station so that nolingering residues from an existing sample cloud 18 can contaminate thetest station area when successive containers C reach the test stationfor sampling. Thus, sample carryover between containers is precluded.The duty cycle for operation of fan 15 is controlled by microprocessor34 as indicated diagrammatically in FIG. 1. Preferably fan 15 iscontinuously operating for the entire time the rest of the system isoperating.

A reject mechanism 28 receives a reject signal from microprocessorcontroller 34 when residue analyzer 26 determines that a particularcontainer C is contaminated with a residue of various undesirable types.Reject mechanism 28 diverts contaminated rejected bottles, as to aconveyor 30, and allows passage of uncontaminated, acceptable bottles toa washer (not shown) on a conveyor 32.

An alternative option is to place the bottle test station downstream ofthe bottle washer in the direction of conveyor travel, or to place anadditional test station and sample and residue analyzing system afterthe washer. In fact it may be preferable to position the test stationand system after the washer when inspecting bottles for somecontaminants. For example, if the contaminant is a hydrocarbon, such asgasoline which is insoluble in water, it is easier to detect residues ofhydrocarbons after the bottles have been washed. This is because duringthe washing process in which the bottles are heated and washed withwater, water soluble chemical volatiles are desorbed from the bottles bythe heating thereof and then dissolved in the washing water. Certainhydrocarbons, on the other hand, not being water soluble, may then besampled by a sampler 22 downstream of the washer, to the exclusion ofthe dissolved, water-soluble chemicals. Therefore, the detection of suchhydrocarbons can be performed without potential interference from otherwater soluble chemicals if the bottles pass through a washer beforetesting.

Referring to FIG. 2 there is illustrated a specific embodiment of adetector system for use with the sampling and analyzing system of FIG. 1wherein like reference numerals refer to like parts. The detector systemof FIG. 2 is also fully disclosed in parent application Ser. No.07/890,863 filed Jun. 1, 1992. As illustrated, a nozzle 16 is providedfor generating an air blast which passes into a container (not shown)being inspected. The air passing through nozzle 16 may be heated orunheated, it being advantageous to heat the air for some applications.Juxtaposed to the nozzle 16 is sample inlet tube 20 including a filter40 at the output thereof for filtering out particles from the sample.Suction is provided to tube 20 from the suction side of pump 82connected through a detector assembly 27.

A portion of the sample (for example, 90-95% of a total sample flow ofabout 6000 c.c. per minute), as described in connection with FIG. 1, isdiverted through a bypass line 24 by means of connection to the suctionside of a pump 46. Pump 46 recirculates the air through an accumulator48, a normally open blast control valve 50, and back to the air blastoutput nozzle 16. A backpressure regulator 54 helps control pressure ofthe air blast through nozzle 16 and vents excess air to exhaust 57.Blast control valve 50 receives control signals through line 50A frommicroprocessor controller 34 to normally maintain the valve open topermit the flow of air to the nozzle.

Electrical power is provided to pump 46 via line 46A coupled to theoutput of circuit breaker 76 which is in turn coupled to the output ofAC filter 74 and AC power supply PS.

The detector assembly 27 in the embodiment of FIG. 2 is an analyzerwhich detects the residue of selected compounds such as nitrogencontaining compounds in the containers being inspected by means of amethod of chemiluminescence. This type of detector is generally knownand includes a chamber for mixing ozone with nitric oxide, or with othercompounds which react with ozone, in order to allow them to react, aradiation-transmissive element (with appropriate filter), and aradiation detector to detect chemiluminescence from the products ofreaction. For example, when NO, produced from heating nitrogen compounds(such as ammonia) in the presence of an oxidant (e.g. oxygen in air),chemically reacts with the ozone, characteristic light emission is givenoff at predetermined wavelengths such as wavelengths in the range ofabout 0.6 to 2.8 microns. Selected portions of the emitted radiation ofchemiluminescence, and its intensity, can be detected by aphotomultiplier tube.

Accordingly, in the system of FIG. 2 ambient air is drawn in throughintake 60 and air filter 62 to an ozone generator 64. Ozone is generatedtherein, as by electrical discharge into air, and is output throughozone filter 66 and flow control valve 68 to the detector assembly 27wherein it is mixed with samples from containers input through intaketube 20, filter 40, flow restrictor 42, and converter 44. The samplefrom intake tube 20 is passed through a converter 44, such as anelectrically-heated nickel tube, in which the temperature is raised toapproximately 800° C to 1000° (e.g., about 900° C.) before being inputto detector assembly 27. Temperatures in the range of 400° C. to 1400°C. may also be acceptable. When nitrogen-containing compounds such asammonia are so heated, NO (nitric oxide) is produced, and the nitricoxide is supplied to the chamber of the detector assembly 27. Compoundsother than NO which may react with 03 and chemiluminescence may also beproduced in converter 44 e.g., organic compounds derived from heating ofgasoline, kerosene, or cleaning residue.

A temperature controller 70 supplied with electrical power through atransformer 72 is used to control the temperature of converter 44.

The samples in the detector assembly 27 after passage through itschamber are output through an accumulator 85 and pump 82 to an ozonescrubber 56, and to an exhaust output 57 in order to clear the residuedetector for the next sample from the next container moving along theconveyor 10 of FIG. 1. (As indicated above, an (optional) fan, not shownin FIG. 2, may be employed to help clear any remaining sample cloud fromnear the sample inlet tube 20.) Outputs from detector assembly 27relating to the results of the tests are output through a preamp 84 tomicroprocessor 34 which feeds this information in an appropriate mannerto a recorder 83. The recorder 83 is preferably a conventional striprecorder, or the like, which displays signal amplitude vs. time of thesample being analyzed.

The microprocessor 34 may be programmed to recognize, as a "hit" or thedetection of a specific residue, a signal peak from a photodetector ofthe detector assembly 27 which is present in a predetermined timeinterval (based on the sensed arrival of a container at the teststation) and whose slope and amplitude reach predetermined magnitudesand thereafter maintain such levels for a prescribed duration.

The microprocessor controller 34 also has an output to a bottle ejector28 to reject contaminated bottles and separate them from bottles enroute to a washer.

A calibration terminal 86 is provided for residue analyzer 27 foradjusting the high voltage supply 26A associated with the detectorassembly. Also provided is a recorder attenuator input terminal 88connected to the microprocessor controller 34 for adjusting theoperation of the recorder. Detector assembly 27 receives electricalpower from the high voltage supply 26A.

Additional controls include operator panel 90 including a key pad anddisplay section permitting an operator to control the operation of thedetector assembly 27 in an appropriate fashion.

DC power is supplied to all appropriate components through DC powersupply 78 coupled to the output of power supply PS.

An optional alarm enunciator 80A is provided for signaling an operatorof the presence of a contaminated container. Alarm enunciator 80A iscoupled to the output of microprocessor controller 34 via output controlline 80C. A malfunction alarm 80B is also coupled to microprocessorcontroller 34 for receiving fault or malfunction signals such as frompressure switch 58 or vacuum switch 87 when pressures are outside ofcertain predetermined limits.

Other safety devices may be provided such as vacuum gauge 89, and backpressure control valve 54 for ensuring proper operation of the system.

Most components of the entire system of FIG. 2 are preferably enclosedin a rust-proof, stainless steel cabinet 92. The cabinet is cooled by acounter-flow heat exchanger 91 having hermetically separated sections91A and 91B in which counter air flow is provided by appropriate fans.

The system illustrated in FIG. 2 has a single detector assembly 27 foranalyzing the sample evacuated into tube 20 and converter or converter44. In most instances this system works quite well to detect eitherhydrocarbons or nitrogen containing compounds. However, sometimes asignal of interest may be hidden, or masked by background NOx (NO orNO₂) signals. Also, background signals, particularly during periods ofrapid variation in background, may result in an indication that acompound of interest is present even though the compound isabsent--i.e., a "false positive". Background NOx could vary due topassage of a fork-lift truck in the plant in the vicinity of the testingapparatus; due to different atmospheres in which some of the bottles tobe tested were stored; or due to traffic outside of the plant and othervarious causes.

In order to avoid false positives and to keep signals of interest frombeing hidden in background signals, a preferred embodiment of thepresent invention includes an improved sample analyzer system, portionsof which are illustrated in FIG. 3. The system is similar in manyrespects to that of FIG. 2; however, converter 44, detector assembly 27,and preamp 84 of the FIG. 2 system are replaced by the components shownin FIG. 3.

With reference in detail to FIG. 3, a sample such as from sample cloud18 emerging from a container to be inspected is evacuated into sampletube 20 and passes through filter 40 and flow restrictor 42. The sampleis then split into two parallel flow lines connected to parallelconverters P1, P2. Converter P1 is preferably a ceramic converter sothat the portion of the sample being heated therein is heated in thepresence of ceramic materials. Converter P2 is preferably a nickelconverter (formed of or containing nickel) so that the portion of thesample being heated therein is heated in the presence of nickel oxide.The output of container P1 is connected to the input of achemiluminescence detector assembly D1 and the output of the nickelconverter P2 is connected to the input of a chemiluminescence detectorassembly D2. Chemiluminescence detector assembly D1 is provided with aquartz output filter--e.g., a 0.19 micron cutoff filter--so thatradiation of wavelengths greater than 0.19 microns emitted bychemiluminescence within detector assembly D1 passes through the quartzfilter to a photomultiplier tube which converts the radiation signalinto an electrical signal having a characteristic shape and amplitudefor each substance to be detected at characteristic wavelengths for therespective substances. The radiation emitted is generated by thechemical reaction of the sample in the detector with ozone gas suppliedthereto in a manner which is well known in the art. In other words,detector assembly D1 detects the presence of substances which emitcharacteristic wavelengths of radiation which will pass through a quartzfilter.

Detector assembly D2, on the other hand, has an infrared outputfilter--e.g. a 0.6 micron cut-off filter--which will selectively passradiation in the infrared (and longer wavelength) region of the spectrumcorresponding to various nitrogen containing compounds.

In operation, a sample from cloud 18 is sucked through sample tube 20,filter 40, and flow restrictor 42 and is split into approximately equalportions to pass into and through the two converters P1, P2 arranged inparallel. Both converters P1 and P2 are heated to approximately 900° C.A temperature of 900° C. is preferable, however, the system will achievesatisfactory results if the converters are heated to substantially thesame temperature in the range of approximately 800° C. to 1400° C.

As stated above, converter P1 is a ceramic converter whose reactionchamber is lined with, a material such as aluminum oxide or the like.The other converter P2 is made of or contains nickel, e.g. converter P2is a nickel alloy tube or a ceramic tube containing a nickel wire coil.

Typical dimensions for the converters P1 and P2 are approximately 15"long; 0.03-0.2" inside diameter; and typically 1/8" inside diameter,1/4" outside diameter.

Detectors D1 and D2 are coupled to the outlets of the respectiveconverters P1 and P2 and to a vacuum in order to draw the sample throughthe converters and the detectors. The outputs of the detectors D1 and D2are connected through suitable amplifiers A1 and A2 to a microprocessorcontroller 34, which includes analog to digital converters and isoperable to process the electrical signals S1 and S2 produced by thedetectors D1 and D2 and provide outputs which include individualprocessed signals and a net signal which results from the subtraction ofsignals output from the respective detectors--e.g., a net signal(S2-S1).

Preferably, the flow resistance through converters P1 and P2, detectorsD1 and D2, and flow lines associated with these components is designedto be relatively equal so that timewise variation of background is thesame as measured by detectors D1 and D2. The main flow resistance (andhence flow rate) is set by the flow restrictor 42.

FIGS. 4-6 show amplitude vs. time graphs of signals output fromdetectors D1 and D2 for various substances to be detected. FIGS. 4-5show signals with relatively high, time-varying backgrounds. Subtractionof signals produces a result (S2-S1) which is a positive (+) signal fordetection of nitrogen compounds, and a negative (-) signal for detectionof hydrocarbons, (olefins, are in diesel fuel or kerosene). The signalis positive for nitrogen containing compound detection (FIG. 4) because,at typical temperatures and flow rates employed, converter P1, which isceramic, does not produce appreciable amounts of NO in the absence ofnickel oxide. The signal (S2-S1) is negative for olefins (FIG. 5)because the signal S1 of detector D1 is larger than the signal S2 ofdetector D2 for at least two possible reasons. The first reason is thatmore signals related to olefin chemiluminescence are produced in theceramic converter than in the nickel converter, and the second reason isthat less of the chemiluminescence radiation produced in detector D1 isfiltered out by the quartz filter of detector D1 than by the infraredfilter of detector D2. In any event, a net negative signal (S2-S1)resulting from the subtraction is an indication of the presence of anolefin contaminant such as diesel fuel or kerosene.

Note that the signal (S2-S1) of FIG. 4 might be missed unlesssubtraction of signals is performed since the high level of backgroundsignals of S2 and S1 might permit the nitrogen compound signal to hide.Also, the importance of avoiding time shifts in flow through theconverters P1, P2 and detectors D1, D2 is apparent, particularly foranalysis under conditions where background levels vary rapidly and wouldnot subtract to zero if time-shifting occurred.

The ceramic converter P1 apparently "cracks" the diesel fuel or keroseneto produce a double-bonded hydrocarbon, most probably an alkene such as1-butane or propylene, which chemiluminesces with ozone (O₃). Apparentlyin the nickel containing converter P2, some of the cracked materialburns or reacts to form non-chemiluminescing compounds before as much ofthe alkene is formed in the nickel containing converter P2 as in theceramic converter P1. This theory would seem to explain the result thatfor gasoline which already contains alkenes, the alkenes are apparentlyunaffected in passing through the converters. In fact, the convertersare usually not necessary to detect gasoline by chemiluminescence.

For gasoline contaminants the net signal (S2-S1) will typically benegative because the quartz filter of D1 attenuates less radiation thanthe infrared filter of D2.

In order to avoid missing other foreign substances such as chemicalsfrom a cigarette in a container or sample being analyzed, the thresholdlevels of the individual signals S1 and S2 should be analyzed inaddition to the net signal (S2-S1). See, for example, FIG. 6, in whichit can be seen that for certain contaminants it's possible thatsubtraction of the signals S2 and S1 yields a net result of (S2-S1)nearly 0, even though each detector contains signal peaks which occur ata specified time (within a window relative to arrival of containers atthe test station) and have a shape and amplitude which satisfy criteriafor the presence of a contaminant. Thus if the peaks of the individualsignals S1 and S2 are compared to threshold levels TL, the contaminant,such as cigarette residue, may be properly detected. The signalsillustrated in FIG. 6 for S1 and S2, respectively, indicate such peaksand therefore the system would indicate a hit and produce an appropriatereject signal. The sharp spike-like signals of FIG. 6 and those of FIG.7 (which are for analysis of a sample illustrated to lack contaminantsof the type and quantity to be detected), though above the thresholdlevel (TL), lack the necessary shape characteristics to indicate thepresence of a contaminant.

Accordingly, the microprocessor controller 34 in accordance with theappropriate software provided for the method of the present invention,not only subtracts signal S1 from S2 to determine the presence orabsence of certain hydrocarbons or nitrogen containing compounds, butmay also look at the individual signals S1 and S2 to determine how theindividual signals compare to the predetermined threshold criteria. Thepossibilities of various detection logic criteria are indicated in thefollowing table.

    ______________________________________                                        Detection By Net Signal                                                                      Indicates            Drawing                                   S2-S1          Presence Of  Action  Figure                                    ______________________________________                                        +   (above + threshold)                                                                          Nitrogen (NH.sub.3)                                                                        Reject                                                                              FIG. 4                                  -   (below - threshold)                                                                          Diesel Fuel, Reject                                                                              FIG. 5                                                     Kerosene                                                   0   (but S1 and S2 Cigarette or other                                                                         Reject                                                                              FIG. 6                                      otherwise show peak                                                                          Contaminants                                                   of specified character)                                                   0   (and S1 or S2  No Contaminant                                                                             Accept                                                                              FIG. 7                                      (or both) lack peak                                                           of specified character)                                                   ______________________________________                                    

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method of detecting selected compounds,including certain nitrogen containing compounds in a sample bychemiluminescent gas phase reaction of nitric oxide with ozone, saidnitric oxide being present as both a background component in the sample,and as a product of the conversion of one of said nitrogen containingcompounds comprising the steps of:collecting the sample; dividing thesample into first and second portions; heating the first portion of thesample to a first predetermined temperature in a first pyrolyzer havingfirst conversion properties such that no appreciable amounts of nitricoxide can be produced therein from conversion of the nitrogen containingcompounds; heating the second portion of the sample to a secondpredetermined temperature in a second pyrolyzer having second conversionproperties such that measurable amounts of nitric oxide can be producedtherein from the nitrogen containing compounds; mixing the heated firstportion of the sample with ozone to cause a chemical reaction therewithin order to generate radiation by chemiluminescence havingcharacteristic wavelengths related to nitric oxide from the backgroundcomponent in said first portion; mixing the heated second portion of thesample with ozone to cause a chemical reaction therewith in order togenerate radiation by chemiluminescence having characteristicwavelengths related to nitric oxide in the background component and alsonitric oxide produced from nitrogen containing compounds in said secondportion which are converted in the second pyrolyzer; selectivelydetecting radiation emitted by chemiluminescence from the first portionof the sample; selectively detecting radiation emitted bychemiluminescence from the second portion of the sample; generatingfirst electrical signals from the radiation selectively detected fromthe first portion of the sample and second electrical signals from theradiation selectively detected from the second portion of the sample;and comparing the first electrical signals with the second electricalsignals in order to determine the presence or absence of selectednitrogen containing compounds in the sample; wherein said heating,mixing, detecting, and generating steps are performed at substantiallythe same times for said first portion as for said second portion andwherein the step of comparing includes the steps of subtracting thefirst electrical signals from the second electrical signals so as tocancel background components of nitric oxide signals in said portionsand comparing the net result to first predetermined threshold criteria.2. The method of claim 1 wherein said first pyrolyzer is formed fromceramic materials and said second pyrolyzer is formed from nickelmaterials.
 3. The method of claim 1 including the further steps ofcomparing each of the first and second electrical signals individuallywith second predetermined threshold criteria.
 4. The method of claim 1wherein said dividing step is performed in a manner to yieldsubstantially equal portions of the sample.
 5. The method of claim 1wherein said heating steps are performed in first and second converterseach heated to substantially the same temperature in the range of about800° to 1000° C.
 6. A system for detecting selected compounds, includingcertain nitrogen containing compounds in a sample by chemiluminescentgas phase reaction of nitric oxide with ozone, said nitric oxide beingpresent as either a background component in the sample or as a productof the conversion of one of said nitrogen containing compoundscomprising:means for collecting the sample; means for dividing thesample into first and second portions; first converter means for heatingthe first portion of the sample to a first predetermined temperature,said first converter means having first conversion properties such thatno appreciable amounts of nitric oxide can be produced therein from thenitrogen containing compounds; second converter means for heating thesecond portion of the sample to a second predetermined temperature, saidsecond converter means having second conversion properties such thatmeasurable amounts of nitric oxide can be produced therein from thenitrogen containing compounds; means for mixing the heated first portionof the sample with ozone to cause a chemical reaction therewith in orderto generate radiation by chemiluminescence having characteristicwavelengths related to nitric oxide from the background component insaid first portion; means for mixing the heated second portion of thesample with ozone to cause a chemical reaction therewith in order togenerate radiation by chemiluminescence having characteristicwavelengths related to nitric oxide from the background component andalso nitric oxide formed from conversion of nitrogen containingcompounds in said second portion; means for selectively detectingradiation emitted by chemiluminescence from the first portion of thesample; means for selectively detecting radiation emitted bychemiluminescence from the second portion of the sample; means forgenerating first electrical signals having amplitudes and durationsrelated to the detected characteristic wavelengths of radiation emittedfrom the mixed first portion of the sample and second electrical signalshaving amplitudes and durations related to the detected characteristicwavelengths of radiation emitted from the mixed second portion of thesample; and means for comparing the first electrical signals with thesecond electrical signals in order to determine the presence or absenceof selected nitrogen containing compounds in the sample; said heating,mixing, detecting, and generating means being operable to process saidfirst portion at substantially the same relative times as the processingof said second portion thereby to permit cancellation of signals relatedto background components of nitric oxide signals from said first andsecond electrical signals by said comparing means.
 7. The system ofclaim 6, wherein the first converter means has a heating chamberincluding ceramic materials and the second converter has a heatingchamber including nickel materials.
 8. The system of claim 6 wherein themeans for comparing is operable to subtract the first electrical signalsfrom the second electrical signals and to compare the net result topredetermined threshold criteria.
 9. The system of claim 8 furtherincluding means for comparing each of the first and second electricalsignals individually with predetermined threshold criteria.